1. Field of the Invention
The present invention pertains to an optical, low-pass filter, anti-aliasing apparatus and, in particular, to an optical, low-pass filter, anti-aliasing apparatus for use with a focal plane array such as an image sensing device, for example, a solid-state image sensing device using a CCD (charge-coupled device) or the like, which produces a predetermined image pickup output by carrying out spatial sampling.
2. Description of the Prior Art
A color video camera typically produces a color signal output corresponding to the viewed object scene by color coding an image of the object scene with a color filter which is disposed at the front of the video camera and by spatially sampling each color using an array of photosensitive elements such as, for example, a CCD, as a solid-state image sensing device. In such an imaging system, it is well known that components in the object scene which contain frequencies too large to be analyzed with the sampling interval used contribute to the amplitudes of lower frequency components and produce, thereby, image errors which are referred to in the art as aliasing distortion or undersampling artifacts.
In general, an optical system for the array can eliminate aliasing if it has a frequency response which cuts out the higher frequency content of the object scene. As a result, it is well known in the prior art that the design of electronic imaging systems involves a trade-off between image sharpness and the susceptibility of the imaging system to aliasing distortions or undersampling artifacts. To effect this trade-off, an optical apparatus such as, for example, a birefringent blur filter has become a common component in consumer color video cameras. Such apparatus are typically placed between a lens and the photosensor array to provide a low-pass filter function which reduces the spatial frequency content of the object scene at frequencies above the Nyquist frequency of the photosensor array to make the imaging system less susceptible to aliasing distortion. For example, for many available sensors wherein equal pixel densities in each of the sensed colors provide that each of the sensed colors have the same Nyquist frequency, an achromatic low-pass filter or "blur" is effective in minimizing aliasing distortion.
An article entitled "Optical Low-Pass Filter for a Single-Vidicon Color Television Camera" by M. Mino and Y. Okano, in Journal of the SMPTE, Vol. 81, April, 1972, pp. 282-285 describes several desirable conditions which an optical low-pass filter used to remove aliasing distortion should satisfy. Specifically, one condition is that the optical low-pass filter should preferably be a phase filter which does not diminish the light level in the transmitted light. Another condition for the optical low-pass filter is that its effect should be independent of the aperture of the optical imaging system. In addition to these conditions, one may add the condition that the optical low-pass filter be easily manufactured at a relatively low cost.
In the art, many attempts have been made to provide apparatus to low-pass filter the spatial frequency of an object scene. However, each of these has at least one drawback. For example, an aliasing suppression phase filter which is commonly used in commercially available video cameras is a birefringent blur filter. Such a filter is typically made of crystalline quartz wherein the crystal axis of the filter plates are oriented at about a 45.degree. angle with respect to the surface. In this orientation, the birefringent quartz exhibits the double-refraction effect, and an unpolarized input ray passing into the filter emerges as two polarized output rays. This type of filter suffers from the drawback that it is rather expensive, and it is also rather complicated to manufacture.
Another example which is well known in the art is an optical noise phase filter which is comprised of statistically distributed phase elements. The optical noise filter suffers from a drawback in that it becomes difficult for the phase elements to be distributed statistically as the aperture stop of the objective lens of the imaging system is reduced.
In addition to the birefringent blur filter and the optical phase noise filter briefly discussed above, it is well known in the art that a phase diffraction grating can be used as a frequency selective filter. However, such a grating is frequency selective, and the size of the grating is restricted by the objective aperture stop. In addition, such gratings are rather expensive and are also rather complicated to manufacture.
As one can readily appreciate from the above, there is a need in the art for an optical, low-pass, phase filter which is effective at substantially all aperture stops of an optical imaging system and which is relatively simple and inexpensive to manufacture.